Spec­tro­scopic Meth­ods Of De­tect­ing Syn­thetic Di­a­monds

By M. D. Sas­try, San­desh Mane, Ma­hesh Gaonkar, Bhavik Joshi, Ra­jen­dra Ardalkar

Solitaire - - CONTENTS -

Dur­ing the re­cent past, there has been tremen­dous progress in lab­o­ra­tory- grown di­a­monds, rang­ing from nano di­a­monds to large sin­gle crys­tals of gem­mo­log­i­cal in­ter­est. The di­a­mond syn­the­sis is done by two routes; one is by high pres­sure high tem­per­a­ture ( HPHT) method and the other us­ing Chem­i­cal Vapour De­po­si­tion (CVD). A ma­jor break­through, which has rel­e­vance to gem­stone in­dus­try, is the large- scale pro­duc­tion of colour­less di­a­monds by HPHT and CVD routes in sizes rang­ing from half a cent to a few carats. This ar­ti­cle deals with the dis­tinct spec­tro­scopic fea­tures ob­served in syn­thetic di­a­monds and their use as di­ag­nos­tic meth­ods for the de­tec­tion of syn­thetic di­a­monds. Fur­ther, it gives an ac­count of de­vel­op­ment of the lu­mi­nes­cence- based method in our lab­o­ra­tory to de­tect and iso­late syn­thetic di­a­monds in large di­a­mond batches (or pack­ets) of nat­u­ral di­a­monds in star- melee sizes.

Di­a­mond, made up of sp3 hy­bridised car­bon atoms, has re­mark­able op­ti­cal, ther­mal and elec­tronic prop­er­ties. Its gem­mo­log­i­cal ap­pli­ca­tions are well known. It is a colour­less solid when free of point de­fects and chem­i­cal im­pu­ri­ties. How­ever di­a­mond can be ob­tained in many at­trac­tive colours due to plas­tic de­for­ma­tion and in­ter­play of ni­tro­gen im­pu­rity and car­bon va­can­cies. Di­a­mond can be formed in na­ture only un­der high pres­sure and high tem­per­a­tures that ex­ist deep in­side the earth; typ­i­cal pres­sure and tem­per­a­tures be­ing in the range of 10 GPa and 2500°C re­spec­tively. Car­bon would ex­ist as graphite at lower pres­sures and tem­per­a­tures. The triple point at which graphite, di­a­mond and a molten con­coc­tion of car­bon can co­ex­ist is around 5000K and 12 GPa. The syn­the­sis in­volv­ing the dis­so­lu­tion of graphite in molten iron and crys­talli­sa­tion un­der suit­able HPHT con­di­tions us­ing a di­a­mond seed crys­tal is prac­tised in var­i­ous lab­o­ra­to­ries; more of­ten nickel cat­a­lyst is added to lower the tem­per­a­ture and pres­sure.

Since the first suc­cess­ful at­tempts of HPHT syn­the­sis by Swedish Gen­eral Elec­tric Com­pany and the first patent for the HPHT process by Amer­i­can Gen­eral Elec­tric Com­pany (GE) in 1955, a lot of progress was made and large-scale pro­duc­tion fa­cil­i­ties have come up. The high pres­sure con­di­tions were achieved us­ing four anvils in tetra­he­dral ge­om­e­try. The pres­sure and tem­per­a­ture con­di­tions were 100,000 at­mos­pheres and 1600°C re­spec­tively.

An al­ter­na­tive route is epi­tax­ial growth by chem­i­cal vapour de­po­si­tion (CVD) on suit­able sub­strate. The sub­strate could be di­a­mond (for ho­moepi­tax­ial growth) or Si/ß-SiC (for het­roepi­tax­ial growth) or a metal­lic sub­stance like irid­ium. The car­bon atomic vapour is pro­duced by dis­so­ci­a­tion of (CH4 + H2) mix­ture un­der mi­crowave or DCarc in­duced plasma or ther­mally dis­so­ci­ated by hot fil­a­ment and it gets de­posited on the sub­strate form­ing di­a­mond struc­ture. The process oc­curs at a pres­sure of 10-200 torr and the sub­strate tem­per­a­ture rang­ing from 7001000°C. Dur­ing the process of the CVD growth dop­ing can be done by ad­di­tion of gases con­tain­ing dopant atoms such as boron or ni­tro­gen. The hy­dro­gen im­pu­rity, how­ever, could al­ways en­ter the lat­tice as it is present in the plasma as a dis­so­ci­a­tion prod­uct of meth­ane or as an ad­di­tive to the hy­dro­car­bon gas.

The di­a­monds grown via HPHT route are pre­dom­i­nantly of type Ib, and more re­cent ones are of type IIb (few cases of type IIa are also re­ported); all CVD di­a­monds are known to be of type IIa. Cur­rently, the most chal­leng­ing as­pect is the de­tec­tion of syn­thetic di­a­monds mixed

The di­a­monds grown via HPHT route are pre­dom­i­nantly of type Ib, and more re­cent ones are of type IIb (few cases of type IIa are also re­ported); all CVD di­a­monds are known to be of type IIa. Cur­rently, the most chal­leng­ing as­pect is the de­tec­tion of syn­thetic di­a­monds mixed in the nat­u­ral di­a­mond sup­ply chain; this is of spe­cial rel­e­vance to the In­dian gem grade di­a­mond in­dus­try.

in the nat­u­ral di­a­mond sup­ply chain; this is of spe­cial rel­e­vance to the In­dian gem grade di­a­mond in­dus­try.

The R&D ef­forts at the Gem­mo­log­i­cal In­sti­tute of In­dia (GII) re­sulted in the de­vel­op­ment of an in­stru­ment which meets this re­quire­ment with a high de­gree of sat­is­fac­tion. In what fol­lows these as­pects will be pre­sented briefly.

HPHT Syn­thetic Di­a­monds

Fourier Trans­form-In­frared (FTIR), Ra­man, Pho­to­lu­mi­nes­cence (PL) and UV-Vis­i­ble ab­sorp­tion meth­ods are con­ven­tion­ally used for di­a­mond diagnostics. Fig­ure 1 shows the spec­tral dif­fer­ences be­tween nat­u­ral di­a­mond and HPHT syn­thetic di­a­monds. The HPHT syn­thetic di­a­mond con­tains pre­dom­i­nantly iso­lated ni­tro­gens at car­bon sites as im­pu­ri­ties (type Ib di­a­mond). The lo­cal mode of such a ni­tro­gen site has ab­sorp­tion at 1130 cm-1 and it is dis­tinctly dif­fer­ent from that due to A and B ag­gre­gates of ni­tro­gen that oc­cur in nat­u­ral di­a­monds. Fig­ure 2 shows the 785 nm ex­cited PL of HPHT syn­thetic di­a­mond. The emis­sion ob­served at 987 nm is due to the H2 cen­tre, elec­tron trapped N-V-N cen­tre. Such cen­tres are ob­served in nat­u­ral crys­tals only af­ter ra­di­a­tion and heat treat­ments, as grown HPHT syn­thet­ics do sta­bilise H2 cen­tres.

CVD Di­a­monds

We have stud­ied the spec­tral prop­er­ties of a num­ber of CVD-grown crys­tals ob­tained from dif­fer­ent sources. While all the syn­thetic HPHT grown di­a­monds are of type Ib or IIb by ad­di­tion of boron, quite of­ten one can grow highly pure type IIa di­a­monds by the CVD

route. Fig­ure 3 shows the FT-IR spec­tra of a typ­i­cal CVD-grown di­a­mond. The spec­trum of nat­u­ral type IIa di­a­mond is in­cluded for com­par­i­son. It can be seen that there are no ab­sorp­tion fea­tures in the ni­tro­gen re­gion (600-1400cm1) in both crys­tals.

Com­pared to HPHT-grown di­a­monds, the de­tec­tion of CVD di­a­monds poses more chal­lenges. The point de­fects in CVD di­a­monds are dif­fer­ent from those ob­served in HPHT di­a­monds. These are sil­i­con-va­cancy cen­tres (SiV and SiV-) and hy­dro­gen re­lated cen­tres such as N-V-H. Fig­ure 4 shows the 514 nm ex­cited PL of a CVD di­a­mond and Fig­ure 5 shows the 785 nm ex­cited PL of the same di­a­mond at 77K. The PL emis­sion from SiV- at 737 nm and that due to SiV0 at 947 nm can be seen in Fig­ures 4 and 5 re­spec­tively.

This work demon­strates the ne­ces­sity of us­ing all the spec­tro­scopic tech­niques in tan­dem to get the com­plete in­for­ma­tion about the di­a­monds.

Lu­mi­nes­cence Method

The lu­mi­nes­cence prop­er­ties of di­a­monds have proved to be in­valu­able in the de­tec­tion of syn­thetic di­a­monds. It is known that all nat­u­ral di­a­monds be­long­ing to Ia (98% abun­dant) and IIa di­a­monds (nearly 2%) do not show phos­pho­res­cence. The ex­cep­tions are those be­long­ing to IIb (abun­dance less than 0.1%). In the flu­o­res­cence mode all nat­u­ral di­a­monds ex­hibit blue flu­o­res­cence (A-band). Among colour­less to near colour­less syn­thet­ics, most of HPHT syn­thet­ics ex­hibit phos­pho­res­cence; among CVD­grown syn­thet­ics some ex­hibit phos­pho­res­cence and most oth­ers show flu­o­res­cence in green/pink or yel­low. Us­ing the lu­mi­nes­cence tech­nique, we have de­vel­oped an in­stru­ment for quickly screen­ing di­a­mond lots typ­i­cally in 0.5 cents to a few cents, for the pres­ence of syn­thetic di­a­monds.

A sum­mary of the per­for­mance of the ma­chine de­vel­oped by GII is given be­low.

Ta­ble I: Present sta­tus of di­a­mond screen­ing of colour­less to near colour­less syn­thetic di­a­monds by GII (Qchk) ma­chine.

Fig­ure 1: FT-IR spec­tra of a nat­u­ral type Ia di­a­mond and a HPHT syn­thetic di­a­mond. The syn­thetic di­a­mond has peaks at 1344 and 1107 cm-1 char­ac­ter­is­tic of iso­lated ni­tro­gens.

Fig­ure 2: 785 nm ex­cited photo lu­mi­nes­cence (PL) of a HPHT syn­thetic di­a­mond at 77K. Strong sig­nal due to H2 cen­tre at 987nm can be seen.

Fig­ure 3: FT-IR spec­trum of a CVD syn­thetic di­a­mond (top).The spec­trum of nat­u­ral type IIa (bot­tom) is in­cluded for com­par­i­son.

Fig­ure 6: Typ­i­cal di­a­mond sam­ple lot in small sizes (0.5 cent to 5 cents).

Typ­i­cal lu­mi­nes­cence of di­a­monds un­der in­ter­band ex­ci­ta­tion. Rows A & B: CVD Row C: Nat­u­ral Row D: HPHT

Fig­ure 5: 785 nm laser ex­cited PL of a CVD syn­thetic di­a­mond. The emis­sion at 947 nm is due to SiV0

Fig­ure 4: 514 nm laser ex­cited PL of a CVD syn­thetic di­a­mond at 77K. The emis­sion due to SiV- at 737 nm can be seen. This is ab­sent in most nat­u­ral di­a­monds.

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